So maybe you heard that sunscreen is basically useless and lying to us all.

Those headlines came after a recent Consumer Reports study tested more than 60 sunscreens labeled SPF 30 or higher and found that 43% failed to live up to their label. So does that mean we should all just give up and not even bother?

Of course not. BuzzFeed Health talked to several dermatologists who were skeptical of the Consumer Reports results, including sun exposure researcher Brian Diffey, PhD, emeritus professor of photobiology at Newcastle University.

"Measuring the SPF of a product is incredibly variable," he told BuzzFeed Health. And there are several factors that influence that variability, including the lab you're testing in, how the sunscreen is applied, the training and expertise of the researchers, and so on. When a product is tested before it hits stores, the SPF is tested multiple times at multiple labs. According to Diffey, there's no reason to think that this one study is more reliable than all of the testing a product undergoes before it's put on the market.

But, okay, if you're not using sunscreen properly, it won't do a whole lot of good.

1.That SPF number probably doesn't mean what you think it means.

You may have heard that SPF (sun protective factor) refers to how long you can stay in the sun without getting burned. So if you typically could be in the sun for 10 minutes without turning pink, then wearing an SPF 15 means you could stay out for 15x as long, explains dermatologist Dr. David Leffell, chief of dermatologic surgery and cutaneous oncology at Yale School of Medicine.

While that's technically true, it's not really the best way to think about it — especially since it assumes you're using enough and reapplying enough, which most people don't, says Leffell.

A better way of understanding SPF is by the percentage of UVB rays it blocks from your skin:

* SPF 15 blocks 93% of UVB rays

* SPF 30 blocks 97% of UVB rays

* SPF 50 blocks 98% of UVB rays

* SPF 100 blocks 99% of UVB rays

2.You're not getting the SPF on the label if you don't use enough sunscreen.

So you wore SPF 30 and reapplied every two hours, and you still burned. You may not have been using enough, which means you weren't actually blocking 97% of UVB rays.

According to Diffey, manufacturers use 2 milligrams of sunscreen per square centimeter when they're testing the SPF of a product. So for someone who's 150 lbs and 5'5, that's about 1.25 ounces of sunscreen (you can find out your own body surface area here). If you go by the shot-glass rule (using a shot-glass-worth of sunscreen for your whole body), that's only 1 ounce. So, depending on your size, you might need a shot glass and a quarter or a shot glass and a half every time you reapply to get the SPF you see on the label.

The problem is, most people use about half of that, which means you're only getting half the protection. "I tell people to go higher on the number, because the reality of people actually using that much is low," Dr. Kavita Mariwalla, NY-based dermatologist, tells BuzzFeed Health.

3.Using a higher SPF doesn't mean you can stay out longer without reapplying.

A higher SPF just blocks a higher percentage of the sun's rays. It doesn't give you permission to stay out longer without reapplying.

"All sunscreen goes away after three hours," says Mariwalla. That's why you need to reapply every two hours to make sure you're really getting the protection on the label.

And FYI: All sunscreen wears off over time — whether you're in the sun or not. So let's say you put on sunscreen at 8 a.m. before you leave the house, then you drive for an hour and get to the boardwalk around 9 a.m., then you grab some breakfast and finally get your chair in the sand by 10 a.m. Yep, it's time to reapply, even though you just got to the beach.

4.You shouldn't use the same sunscreen in Miami that you would use in New York.

UV rays are stronger the closer you are to the equator, so you'll want to use a higher SPF there. "An SPF 30 in Norway is going to give you a lot of more protection than the same SPF in Aruba," says Mariwalla.

5.If you're just spraying on your sunscreen, you're doing it wrong.

"People rely on sprays much more than they should," says Mariwalla. Sure, they're great for hitting that spot you can't reach, or for quickly applying lotion to kids, but...wait for it...

You still need to rub them in.

Sunscreen works best when you have as uniform a layer as possible, says Diffey. So you can't just spray all over yourself and call it a day. Mariwalla suggests spraying two inches from your skin, then spreading it so that it's evenly covering every inch of skin.

6.Yes, there is a difference between SPF 50 and SPF 100.

If SPF 50 blocks 98% of rays and SPF 100 blocks 99% of rays, do you really need that extra 1% of coverage? Well, maybe.

Think of it this way: SPF 50 is letting through 2% of rays, while an SPF 100 lets through 1%. By that ratio, Diffey explains that the SPF 50 is letting through double the amount of rays as an SPF 100. And if you burn easily, that can make a difference. And, again, this is assuming you're using the right amount of sunscreen.

7.You can rack up skin damage even without a sunburn.

So what if you're not really a lays-out-until-crispy person? Even if it doesn't cause a burn, prolonged sun exposure can damage your skin cells and make them unstable, putting you at risk for melanoma and other skin cancers, says Mariwalla.

Along those lines, make sure your sunscreen protects against both UVB and UVA rays (it should say "broad spectrum" on the label). UVB rays are typically thought of as the ones responsible for burns, while UVA rays are more involved in signs of aging (like wrinkles). But we now know that UVA and UVB represent a spectrum, and the spectrum overlaps, explains Leffell, so both types of rays can play a role in skin cancer.

8.You should always reapply after swimming — regardless of how "water-resistant" your sunscreen claims to be.

Especially if you're splashing around or rubbing your eyes as you come up out of the water, because you're wiping off a lot of product. Plus, the water kicks up a ton of UV rays, says Mariwalla, which explains why you fried your butt on a paddle board that one time.

So just ignore that whole "water resistant up to 80 minutes" label and reapply as soon as you towel off.

9.Yes, people of color need to use sunscreen, too.

Sun damage can manifest differently depending on your skin's pigmentation, so heavy sun exposure may not result in a "sunburn" if you have darker skin. But that doesn't mean you're not exposing your skin cells to damaging UV rays, explains Mariwalla.

Everyone — regardless of their skin color — should be wearing sunscreen when they're out in the sun, says Mariwalla.

10.Sunscreen won't give you a vitamin D deficiency.

This can be a pretty contentious topic, but the experts we talked to all agreed: You don't need to forgo sunscreen in order to get vitamin D. You can get vitamin D from walking to get lunch and back, or you can get it from vitamin-enriched foods or supplements. You don't need to lay SPF-less in the sun or a tanning bed to get enough.

11.Sunscreen isn't the only way to prevent skin cancer.

Your sunscreen should just be one thing in your skin-cancer-preventing arsenal. Good sun protection also includes covering up well, hanging out in the shade, and planning your day so you're not spending all your time outside during the hours when the sun is the strongest.